Thats a tough one. Try this, get insurance on the trailer first. Shouldn't be a problem if the seller sends you the reg number, or just tell the insrance agent that you custom built the trailer yourself. (Done that before).
As for the boat, make all of the arrangements and try one of two things........have him send you a bill of sale, then get the boat licensed in Canada, then go down and pick it up...on your way back you can say your coming back from a fishing trip. Or......try and have the guy meet you in Canada and do everything here!
Just a couple of thoughts, haven't done it myself but probably what I am going to do when I get a new boat!